The present invention relates to loudspeaker enclosures and, more specifically, to loudspeaker enclosures which minimize distortion caused by phase nonlinearities.
The field of loudspeaker design has been extremely active in the last decade due to the advances made in stereo amplification and sound pick-up equipment. However, although the loudspeakers themselves have undergone great advances, the speaker enclosures have not kept pace with the loudspeaker improvements. Many speaker enclosures are merely soundproof boxes formed of furniture-grade wood and with acoustic padding lining the inner surface. Although acoustic-suspension enclosures are the most popular, the base-reflex cabinet is still being made, and the main departure from the older well-known enclosures is a removable front grill formed of cellular foam.
A major problem presented by these conventional speaker enclosures is time-delay distortion, which is based upon phase nonlinearities, and which results in the sound being "smeared". This smear is caused by an uneven delay of different parts of the audio spectrum. In other words, in a normal acoustic situation, the upper harmonics of a piano note will arrive at the listener's ear a fraction of a second later than the fundamental tone. To overcome this time-delay distortion, it has been proposed to use what are known as stepped cabinets, which consist of individual boxes arranged in a stacked relationship. The speakers are then not aligned in the same vertical plane. Because the tweeter is a different distance from the listening point than is the woofer, the delay problems are mechanically corrected. However, the stepped cabinets present a problem with sound diffraction, a phenomenon which occurs when a sound wave reaches an obstruction or a surface which drops away quickly. These stepped cabinets have many sharp edges and corners involved in the arrangement of the stacked boxes.
Other problems encountered in conventional loudspeaker enclosures are vibration of the baffle or soundboard caused by mechanical coupling from the loudspeakers or the drivers. Additionally, since most speaker enclosures are intended to rest or sit on the floor, the flooring material acquires far too much importance in the sound reproduction system. Another problem encountered in conventional enclosures relates to the extent of dispersion which is available, specifically, in the higher frequencies which tend to be very directional.
All of the above drawbacks in loudspeaker enclosures tend to produce what is called "coloration" of sound. In other words, the speaker enclosure tends to inject its own personality into the sound and, hence, colors the sound presented to the listener. While this is not a fatal flaw, it is a drawback, since the loudspeaker and its enclosure should merely reproduce the sound and not influence it or "color" it.